1. Field of the Invention
The present invention relates generally to a capstan servo system for a video tape recorder which will be hereafter referred to as VTR. More specifically, the invention relates to a capstan servo system which permits assemble edits without any transient error at editing points.
2. Description of the Prior Art
Conventional VTRs, of the type having one or more rotating heads which scan the video tape at an angle with respect to the direction of advancement thereof, can be adapted to be set into an editing mode so that a new scene is recorded sequentially and contineously after a previously recorded scene. In such a mode, sometimes called as and assemble mode, the VTR is stopped after one scene is recorded, and another scene is recorded immediately after the one scene. In another mode, simetimes called in insert mode, a new scene is recorder between two scenes which have been recorded on the tape.
In either of the insert and assemble modes, the edit points, i.e., the points connecting the successive scenes, are determined, for example, by an operator viewing the video picture on a monitor. During a usual edit operation, the operator stops the VTR at a selected point by depressing a pause button. When the operator is ready to record the new video material, the VTR is released from its pause mode and is set into its record mode, and a new video scene is recorded begining at the edit point.
Whenever such assemble or insert recording operation is performed, care must be taken to prevent the video signal from generating transisent error at the edit point. In order to avoid such transient error, there is known such capstan servo systems for the VTR.
In a known capstan servo system, a pair of frequency generators are employed for monitoring revolution of a capstan. Each of the frequency generators are adapted to produce a signal having a frequency propotional to the rotation speed of the capstan. This signal will be hereafter referred to as "capstan speed indicative signal". The pair of frequency generators generate capstan speed indicative signals in different phases. The capstan speed indicative signals are input to a well-known speed control circuit having a frequency-to-voltage (F/V) converter. The speed control circuit derives an output voltage for controlling rotation speed of the capstan. The output of the speed control circuit will be hereafter referred to as "capstan speed control voltage". The capstan speed control voltage from the speed control circuit is fed to the capstan motor through an operational amplifier for maintaining the capstan speed constant. Therefore, the capstan motor, the frequency generators, the speed control circuit and the operational amplifier constitute a closed loop for allowing feedback control of the capstan speed.
On the other hand, during reproduction, a reproducted control signal (30 Hz) PB-CTL which is recorded on a tape longitudinally and a reference signal REF of 30 Hz which also serves as a reference for controlling rotation of a head drum motor are input to a phase control circuit which has a comparator for comparing the phases of the reproduced control signal and the reference signal. The comparator thus derives an output serving as a phase-error signal. The phase-error signal from the comparator is input to the opperational amplifier as another input therefor. By this. the phase of rotation of the capstan motor is controlled so that the PB-CLT and REF signal conicide.
The capstan servo system in the prior art also have a counter for counting up the capstan speed indicative signals. During recording (REC), a 30 Hz subharmonic signal indicative of the counter value of the aforementioned counter is applied as a replacement of the reproduced control signal PB-CTL to the phase control circuit through a change-over switch.
In the case of assemble editing (ASS), the tape is rewound to the point slightly before the editing point. Then, the tape is advanced or fed to the editing point in the reproduction or playback (PLAY) mode. The operation mode is switched at the editing point from the reproduction mode (PLay) to the recording mode (REC). In the prior art, the counter is adapted to be reset by the reproduced control signal PB-CTL for forced synchronization in reproduction mode. By this, after the editing point, the counter is self-reset or self-triggered to hold phase information for PB-CTL. Therefore, track will not be discontinuously disrupted at all before and after the editing point.
With regard to such prior art capstant servo system, it is organized on an assumption that the frequency of the capstan speed indicative signal is an integer factor of the reproduced control signal PB-CLT (30 Hz). When the capstan speed indicative signal have frequency not integral of the frequency of the reproduced control signal, continuity in the track of the rape can not be maintained before the after the editing point since the subharmonic output of the counter tends to be varied to the frequency, e.g. 31 Hz, other than the frequency of the reproduced control signal, upon switching from the force-reset condition to the self-triggered condition in response to the reproduced control signal PB-CTL. In order to maintain high servo control accuracy, it is required to machine the mechanical component included in the servo loop, especially in the diameter of the capstan. For instance, if the diameter of the capstan is larger than the standard, even though the tape is moving at a constant speed, the frequency of the capstan speed indicative signal will necessarily become lower, whereby the subharmonic output of the counter will be less than 30 Hz. Also, in such prior art capstan servo system, it is required to produce the frequency generators with high accuracy.
Furthermore, since the phase servo system is maintained inoperative in recording mode (REC) or in assemble mode (ASS), it also encounters another problem that the tape feed rate will deviate from the standard rate due to the capstan diameter machining errors and temperature conditions or secular changes in the speed control circuit.